1. Field
Circuit patterning and more particularly to masks used to pattern light sensitive material on substrates or wafers.
2. Background
Patterning is the series of operations that results in the removal of selected portions of surface layers added on a substrate, such as a wafer. Patterning creates the surface parts of devices that make-up a circuit. One goal of patterning is to create in or on the wafer surface, the parts of the device or circuit in the exact dimensions (feature size) required by the circuit design and to locate the parts in their proper location on the wafer surface.
Generally, patterning is accomplished through photolithography techniques. For example, photolithography may be a multi-operation pattern transfer process wherein a pattern contained on a reticle, photo mask, etch mask, or multi-layers mask is transferred onto the surface of a wafer or substrate through a lithographic imaging operation, and a light sensitive material (e.g., photoresist) is developed on the wafer. One goal of circuit designers is to reduce the feature size (the critical dimension) of devices of a circuit, i.e., reduce the smallest feature patternable. A reduction in wavelength of light used in patterning will reduce the critical dimension. Thus, the patterning wavelength can be reduced to under 200 nanometers, and can lie in the extreme ultraviolet (EUV) light region to reduce the critical dimension to 100 nanometers or less.
In the general course of patterning, the image of a reticle or photo mask is projected onto a wafer or substrate surface by an imaging system. EUV light radiation, however, does not pass through quartz or glass, and is therefore typically projected using reflective optics. For example, a reticle or photo mask for EUV light patterning of a light sensitive material may include a multi-layer mask that is created by forming light absorbing material on certain portions of a substrate covered with multiple layers of a reflective material (e.g., a patterning mask). The substrate having only multiple layers of reflective material may be referred to as a “mask blank” (e.g., such as a substrate having multiple layers of reflective material, prior to forming the light absorbing material).
It is important to be able to inspect an EUV light mask blank for defects that may cause errors in the imaging or patterning of the light sensitive material, such as by causing unwanted variations in the image of features (e.g., such as critical features) patterned on that material by the patterning mask formed from that mask blank. Specifically, because of the wavelength of EUV light used to expose the light sensitive material, a small bump with a height as low as two nanometers on the surface of a multi-layered mask blank may cause errors in the imaging or patterning of the light sensitive material, and thus be a defect in the mask blank. Therefore, EUV light patterning photo mask blanks may be inspected during manufacture, after manufacture, prior to shipment, or after shipment to detect “critical defects” (e.g., such as defects that may cause an error in patterning) while minimizing detection of “false defects” (e.g., such as defects that do not cause errors in patterning substantial enough to affect the critical dimension of features to be formed).